The present invention relates to a video signal processing apparatus, a video signal processing method, an edge enhancement apparatus, and an edge enhancement method.
Televisions are equipped with an edge enhancement circuit to increase image sharpness. The most popular technique for edge enhancement is to produce second-order differential signals from a video signal, apply amplitude adjustments to the differential signals, and add the amplitude-adjusted differential signals to the video signal. The resultant video signal has steeper edge portions with overshoot and undershoot, thus apparently having increased sharpness.
Edge enhancement with a second-order differential signal with a large peak-to-peak amplitude (with no amplitude adjustments) excessively enhances a steeper edge portion, resulting in lowered image quality. Images that suffer lowered quality most are those with symbols, letters, thin lines, etc., and on-screen displays (OSD).
There are two known techniques to overcome such a problem. One is to limit second-order differentials signal at a certain level to avoid excessive edge enhancement (disclosed, for example, in Japanese Unexamined Patent Publication No. 2002-290773, referred to as document 1, hereinafter). The other is, that, when a second-order differential signal produced from a video signal at a target pixel reaches or exceeds a reference level, second-order differential signals from the target pixel and several pixels adjacent thereto are cancelled which otherwise be added to the video signal at the target pixel, thus no overshoots being is applied (disclosed, for example, in Japanese Unexamined Patent Publication No. 2000-244775, referred to as document 2, hereinafter).
Another problem to lower image quality is variation in the intermediate gradation level of images with monotonous increase or decrease in gradation per vertically adjacent two lines, such as symbols and letters in an electronic program guidance (EPG) in digital broadcast, when edge enhancement is applied. This problem may be solved by adding video signals at vertically adjacent two lines and averaging the added signals (disclosed, for example, in Japanese Unexamined Patent Publication No. 2004-194044, referred to as document 3, hereinafter).
The above-discussed OSDs with symbols, letters, etc., having sharpened edges are mostly shown in high-vision equipment, such as, D-VHS (a registered trademark) digital VTR (Video Tape recorders), HDD (Hard Disc Drive)/DVD (Digital Versatile Disc) recorders equipped with a high-vision tuner, etc.
Edge enhancement on OSDs with symbols, letters, etc., having sharpened edges causes thicker edges, ringing, etc. The techniques described above are still not enough for edge enhancement on the OSDs.
In detail, limitation of second-order differential signals at a certain level in the document 1 could diminish the effects of edge enhancement on images displayed with an OSD. Cancellation of second-order differential signals at a target pixel and pixels adjacent thereto in an OSD in the document 2 in which edge enhancement is on or off per pixel, could cause discontinuity in the spatial domain or noises when it is on or off in the temporal domain. Moreover, addition of video signals at vertically adjacent two lines and averaging the added signals in the document 3 could cause the same problem as the document 2 due to filtering per vertically adjacent two lines.
Problems related to edge enhancement on OSDs with the techniques described above are further discussed.
Natural images suffer noises when photographed. In contrast, digitally produced OSDs suffer almost no noises, in addition, reach a monitor with completely no noises when supplied through HDMI digital transfer. The HDMI (High-Definition Multimedia Interface) provides high-definition and noiseless video signals, in compliance with the specifications for baseband digital video/audio input/output interface.
Basically, OSDs offer sharpened letters or symbols having steep edges, with no edge enhancement. It is thus better to display images on screen with no edge enhancement if the images are noiseless and already have steep edges.
For such OSDs, the known techniques have the following disadvantages:
Limitation of second-order differential signals at a certain level in the document 1 could diminish the effects of edge enhancement on steep edge portions only in an OSD while the effects remains overall which produce blurry background displays to OSD letters, symbols, etc.
Cancellation of second-order differential signals at a target pixel and pixels adjacent thereto in the document 2 cancels edge enhancement on overshoot portions of a video signal while the effects remain on undershoot portions, thus effective only on edge portions having a large peak-to-peak amplitude. Moreover, having edge enhancement on or off per pixel in the document 2 could cause that it is off on the image portions to which this effect should be applied.